Large electrode with thermal stress relief



July 15, 1952 C. H. CHAPPELL LARGE ELECTRODE WITH THERMAL STRESS RELIEF Filed Oct. 26, 1948 INVENTOR 9 CHARLES H. CHAPPELL ATTORNEY RADIUS 4 INCHES Patented July 15, 1952 LARGE ELECTRODE WITH THERMAL STRESS RELIEF Charles H. Chappell, Niagara Falls, N. Y., jassignor, by mesne assignments, to Union Carbide and CarbonCorporation, a corporation of New York Application October 26, 1948, Serial No. 56,647

' 23 Claims.

The problem of thermalstress in furnace electrodes has been recognized for over 35 years and has become more troublesome as electrodes with increased diameters have been produced. These stresses are due to non-uniformity in temperaturethroughout the electrode and are greatest on its immersion in a hot furnace and upon its withdrawal. Contraction of the outer surface especially'ofan' extruded electrode on cooling imposes. tensional stresses which cause spalling and longitudinal cracks. A longitudinal crack may change direction to a fatal transverse crack.

One early suggestion for reducing thermal stresses is found in the patent-l to Hinckley l,058,057,-April 8, l9l3,in which alongitud'inal slot extended aILthe-way through a radial wall of an electrode provided witha central bore. One objection to this proposal is the dangerof alongitudinal crack oppositethe slot developing from the clamping stress to which the electrode is subjected from its holder. Another objection to that proposal is the danger of rupture from the nipple pressing against the socket when electrodes are connected end to end by the usual threaded carbon or graphite nipple. Since that time electrodes have increased in size and the time of maximum internal stress is believed to be no longer on starting but in stopping the operation of the electrode and on itswithdrawal into the air. At that time the inside of the electrode has been heated to a high temperature and after the outer portionhas been cooled for a minute or two, a steep thermal gradient is produced adjacent the outer surface imposing a tensional stress around the circumference of an electrode transverse cross section tending to form a longitudinal crack.

A large majority of the large size furnace electrodes are of the extruded type in which cracks are practicallynever. noted to start in a transverse direction but begin in .a longitudinal direction and then may change into a transverse direction to. be moreserious and objectionable in nature. Molded electrodes tend to have a smallertransverse coeflicient of expansion and therefore lower transverse stresses, the longitudinal direction containing the larger coefficient of expansion, wherefore. the danger of longitudinal cracks developing. in molded electrodes is usually less than in extruded electrodes. ,By carbon is means either carbon or graphite.

This invention relates to large carbon electrodes, especially those of the extruded type, for electric furnaces and has-for an object to reduce the danger of longitudinal cracks.

Another object is to reduce the thermal stress developed within the electrode, particularly on cooling.

According to this invention, one or more narrow slots extend longitudinally and deep enough to lessen the degree of such thermal stress but shallow enough to leave a substantial part of the radial depth of the electrode wall, thereby providing a firm foundation to resist the tendency to fracture possessed by an electrode having a slotradially all the way through its wall. These narrow slots are preferably filled with a yieldable conductive carbon to lessen the likelihood of rapid oxidation of the carbon side walls. Preferably, both inside and outside radial slots are provided, symmetrically arranged with the inside slots angularly displaced from the outside slots. The outside slots have been found to be the more important slots. However, the inside slots reduce the formation of high compressive stress on the inside and are thus useful in alleviating the danger of excessive tensional stress on the outer portion .of an electrode.

Referring to the drawings:

Fig. 1 is a perspective partly in section of a preferred embodiment of an electrode under, the present invention;

Fig. 2 is an end view of a cylindrical electrode having only outside slots;

Fig. 3 is an end view of a cylindrical electrode having only inside slots;

Fig. 4 shows the calculated temperature gradient in an electrode of 9 inch radius.

. Referring to the drawing, a large cylindrical carbon electrode I0 for use in an electric furnace is provided with three or, more symmetrically arranged and substantially equally, angularly spaced, generally radial outside slots H for the purpose of reducing tensional stress in the outer portions of an electrode cross section. These slots II have a depth of from .1 to .5 or more of the radial depth of the electrode and a width of /8" to perhaps an inch the bases of the slot being rounded as illustrated to minimize stress concentration. The maximum figure named is arbitrary since the slots should not be so wide as to substantially reduce the transverse cross sectional area of the electrode thereby impairing its conductivity. The minimum width suggested for the slots H is determined by the manner in which the same may be formed in manufacture. It is understood that the width of these slots should be large enough to prevent the side walls from touching when the electrode is placed in a hot furnace. symmetrically spaced angularly from the external slots H are' an equal number of internal slots 12 radiating from a central bore l3. In cooling it has been found that these inside slots tend to reduce the compressive stress in the heated central portion of the electrode and therefore also tend to reduce the formation of dangerous tensional stress in the outer portions of the electrode. An axial bore I3 is usually provided when internal slots l2 are provided. The diameter of this bore 13 may vary from .1 to .6 or more of the electrode diameter though the small size bore is preferable in order not to reduce the conductivity of the electrode excessively.

The slots 12 may be from .005 to 1" in Width. Their minimum width should be such that, on cooling, the side walls of the slot 1 2 arenot closed. The upper limit for the slot width-is such that the conductivity of the electrode is not unduly reduced. Here again, the base of each slot 12 is rounded for strength. It will be seen from the drawing that each internal slot I2 is spaced 45 from an external slot H.

The transverse cross-sectional area of the electrode has been reduced by both slots 1 I and 12 less than and preferably between 2% and 7%. The inner slots l2 should reduce the cross-sectional area of the electrode by less that 4%, exclusive of axial bore, and the outer slots by less than 4.4% in order that the conductivityof theelectrode may not be impaired. These values represent what has been found to be good .practice. The problem vsolved by this invention does not exist in small electrodes of less than two inches diameter. As shown in the drawing thedepth of the inner slots is preferably slightly shorter than the depth of the outer slots.

The embodiment illustrated in Fig. 2 is a large cylindrical, electrode having only the external slots Ha of the size and extent indicated for the slots I l in Fig. 1.

The electrode of Fig. 3 contains only the internal slots l2a radiating from the axial bore I3a and of the size mentioned for those slots l2 in Fig. 1.

The slots of each embodiment and the axial bore of Figs. 1 and 3 preferably are each filled with an inert conductive material of a soft and pliable nature to prevent the access of air or oxidizing gases to the slot walls yet to permit movement of the side walls of each slot toward and from one another. Materials suitable for such filling includegraphite, bituminous coal, foundry coke, petroleum coke, sawdust or other carbonaceousmaterial, decomposed carborundum, or mixtures of these materials, each crushed and mixed with any suitable binder. A suitable filler composition is a mixture of 23 parts tar with 77 parts graphitized lump foundry coke crushed to V4" size and smaller.

The tangential stress on the surface of an electrode 18 inches indiameter has been calculated tobe 163 pounds per square inch when the surface temperature is 1600" C., when the electrode is carrying 30,000 amperes and is at equilibrium thermally. Under these conditions the thermal gradient is only 117 C. and such is shown by the curve I 4 in Fig. 4. It has also been calculated that if the same electrode is uniform in temperature and carries no current but is exposed toroom temperature, the stress in the outer fibers will reach 735 pounds per square inch after one minute exposure and 805 pounds per square inch after two minutes exposure. The radial temperature gradient under these conditions was calculated to be 320 C. and 375 C. respectively. The curve l5 in Fig. 4 indicates what is believed to be a temperature gradient for the electrode one minute after it has begun cooling in the air if the conditions of curve 14 had not existed. 'It will be noted that the steepest portion l6 of this gradient is in the outer two inches of the electrode radial depth. Curve H is a composite of curves l4 and I5. Curve: I1 is believed to give an idea of the total temperature gradient .in an electrode.

1Where. external slots are used to reduce the stress in the outer portion of the electrode it will be apparent that such slots should be at least two or three inches in depth in order that they-- mayextend through the portion of the electrode where the steepest temperature gradient occurs. The maximum estimated stress is about 900 pounds per square inch in tension, whichv is 50% greater: than the normal-trans-, verse strength usually reportedfor this type of graphite at room temperature. The neutral stress zoneis believed to be .575 of the radius from the center'in a'solid cylindrical electrode. Although the coefficient .of expansion, modulus of elasticity, elastic limit, etc. of electrode materials are known at.room temperature, these values are not as well known at the high temperaturesto which an electrode is subjected and their, accurate measurement at these temperatures isdiflicult. V e p Whatis claimed is: i v

,1. In a. cylindrical carbonaceouselectric furnace electrode having a diameter of at least several inches, the combination therewith of means for reducing the tendency for a longitudinal crack to form due' to -a substantial temperature gradient between the inner andvouter portions of the electrode, .said means including the provision. in the electrode of at least one generally radial slot extending longitudinally thereof but of less depth than the radial thickness of the electrode, said slot being adjacent at least one of an inner and an outer portion of the radius and of a width to prevent-the side walls of the slot closing .under substantial pressure due to extremes in temperature encountered withinthe electrode, the total reduction in cross sectional area of .the electrode due to said slot being not more than 7percent, said slot being free of. any material which would appreciably impede movement of its side walls toward each other due to temperature changes and free of any material contaminating a bath in which the electrode is adapted to be placed, the unslotted portion of the electrode radially beyond said slot having a substantial depth and providing a foundation against which any clamping stress of an electrode holder may be exerted.

2. An electrode according to claim 1 in which said atleast one longitudinal slot is in a radial inner portion of the electrode but outside of an axial bore.

. 3, An electrode according to claim 2 in which the radial inner. slotted portion contains at least three substantially equally spaced slots of a size reducing the transverse cross sectional area of the electrode by no more than about 4%, exclusive of said axial bore.

4. An electrode according to claim 1 in which said at least one longitudinal'sl'ot is in an outer portion of the electrode and opens onto an outer circumference and an unslotted portion of the electrode is radially inside of said slot.

5 An electrode according to claim 4 in which the externally slotted portion contains at least tional area of the electrode by no more than 4.4% and the basesof said slots are rounded to better withstand thermal stress tending to move the side walls of the slot toward and from one another.

6} An" electrode according to claim 1 in which an axial bore is provided with said at least one radial slot extending outward from said bore and at least another generally radial slot of less depth than the electrode radial wall extends inwardly from the periphery, said slots being angularly displaced from one another, not substantially overlapping radially, and ,a substantialportion of the electrode wall between said slots being unslottedboth radially and angularly.

7. An electrode according to claim 1 in which there are at least three but not more than eight generally radial slots substantially equally spaced angularly. v

8. In a generally cylindrical, uniform and homogeneous electric furnace electrode having a diameter in excess of threeinches, the combination therewith of the improvement for reducing the tendency of a longitudinal crack to form due to a substantial temperaturegradient between the inner and outer portions, said improvement including the provision of at least one generally radial slot extending longitudinally of the electrode of less depth than the radial thickness of the electrode, having a width to prevent the side walls of the slot closing under substantial pressure due to temperature differences in the electrode, of a width effecting a total reduction in transverse cross sectional area of the electrode due to. the, slotted portionof not more than 7 percent and the slotted portion is free of any material which would significantly impede movement of its side walls toward each other due to temperature differences within the electrode and free of any material contaminating a bath in which the electrode is adapted to be placed, the unslotted' wall of said electrode radially beyond said slot being of a radial depth providing a foundation against which a clamping stress may be exerted, the slotted portion being filled with a yieldable carbonaceous conductive material whereby the electrode is integral structurally except for the material filling said slot.

9. An electrode according to claim 8. in which the slotted portion reduces the cross sectional area of the electrode between 2% and 7%.

10. A cylindrical electric furnace electrode having an outside diameter in excess of 8 inches provided with at least three .angularly spaced slots extending inwardly from the periphery, a

distance of .1 to .5 of the radial thickness of the electrode, extending longitudinally throughout at least a major portion of the length of the electrode from one end thereof, each slot being of a width to prevent theside walls touching under pressure on immersion of a cold electrode in a hot furnace, the slotted portion effecting a reduction in transverse cross sectional area of the electrode due to the slots which does not exceed 10%, said slots being free of any stiff material appreciably impeding movement of the side walls toward each other and free of any material contaminating a bath in which the electrode is adapted to be placed, the unslotted portions of the electrode providing a foundation against which a clamping stress of an-electrode holder may be exerted.

"11." In a cylindrical electrode for an electric furnace and having an outside diameter greater than eight inches, the combination therewith of the improvement for reducing the danger of a longitudinal crack being formed due to a temperature gradient between the inner and outer portions of the electrode, said improvement comprising a longitudinal non-axial recess extending throughout at least the major portion of the electrode length from one end thereof, of a width and depth capable of allowing slight relative movement between opposite sides of said recess toward and from one another under stress due to a temperature difference between the inside and outside of said electrode,v the transverse cross sectional area of said electrode being reduced less than 10% by said recess, a substantial portion of the electrode radial depth one side of said recess constituting an imperforate foundation against which a clamping stress of an electrode holder may be exerted. 12. A cylindricalcarbonaceous electric furnace electrode having a diameter in excess of eight inches and having the improvement for reducing a tendency for a longitudinal crack to form due to a substantial temperature difference between inner and outer portions of the electrode, said improvement including at least one generally radial slot extending longitudinally and of less depth than the radial thickness of the electrode, the total cross sectional area of any 'andall of said generally radial slots of the electrode being no more than 7% of the cross sectional area of the electrode, said slot being free of rigid material which would impede movement of its side walls toward each other due to temperature changes and free of any salts contaminating a bath in which said electrode is adapted to be placed.

13. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of eight inches and having the improvement for reducing a tendency for a substantial crack to form due to a substantial temperature difference between inner and outer portions of the electrode, said improvement including at least one generally radial slot extending longitudinally and of less depth than the radial thickness of the electrode, the total cross sectional area of any and all of said generally radial slots of the electrode being between 2% and "1% of the cross sectional areaof the electrode, said slot being free of rigid materialv which would impede movement of its side walls toward each other due to temperature changes and free of any salts contaminating a bath in which said electrode'i s adapted to be placed.

14L A cylindrical carbonaceous electric furnace electrode having a diameter in excess of three inches and having the improvement for reducing a tendency for a longitudinal crack to form due to a substantial temperature difference between innerand outer portions of the electrode, said improvement including outer slots having a cross sectional area of not over 4.4% of the cross sectional area of the carbonaceous electrode and extending less than the radial depth of the electrode.

15. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of three inches and having the improvement for reducing a tendency for a longitudinal crack to form due to a substantial temperature difference between inner and outer portions of the electrode, said improvement including inner slots having a cross sectional area of not over 4% of the cross sectional area of the carbonaceous electrode around a central bore and less depth than the radial depth of the electrode.

16. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of three inches and having the improvement for reducing a tendency for a longitudinal crack to form due to a substantial temperature difierence between inner and outer portions of the electrode, said improvement including radially inner and outer slots of substantially the same width and having a cross sectional area of not over 10% of the cross sectional area of the electrode around a central bore.

17. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of three inches and having the improvement for, reducing a tendency for a longitudinal crack to form due to a substantial temperature difierence between inner and outer portions of the electrode, said improvement including radially inner and outer slots having a cross sectional area of not over 10% of the cross sectional area of the electrode.

18. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of three inches and having the improvement for reducing a tendency for a longitudinal crack to form due to a substantial temperature difierence between inner and outer portions of the electrode, said improvement including outer slots having a cross sectional area of not over 4.4% of the cross sectional area of the carbonaceous electrode and a depth of .1 to .5 of the radial depth of the carbonaceous electrode.

19. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of eight inches provided with means to reduce a tendency for a longitudinal crack to form due to a substantial temperature difference between inner and outer portions of the electrode during or after use, said means including at least one generally radial slot extending longitudinally, said slot being deep enough for its side walls to be free to move toward and from each other due to tem perature differences within the electrode, the depth of said slot being substantially less than the radial depth of the electrode exclusive of any core or bore.

20. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of eight inches provided with means to reduce a tendency for a longitudinal crack to form due to a substantial temperature difference between inner and outer portions of the electrode during or after use, said means including at least one generally radial slot extending longitudinally, said slot being deep enough for its side walls to be free to move toward and from each other due to temperature differences within the electrode, the depth of said slot being substantially less than the radial depth of the electrode exclusive of any core or bore, but the slot depth not extending beyond a neutral stress zone due to said temperature difference within the electrode.

21. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of eight inches provided with means to reduce a tendency for a longitudinal crack to form due to a substantial temperature diiference between inner and outer portions of the electrode during or afteruse, said means including at least one gen erally radial slot extending longitudinally, said slot being deep enough for its side walls to be free to move toward and from each other due to temperature differences within the electrode, the depth of said slot being substantially less than the radial depth of the electrode exclusive of any core or bore, said slot being open radially outwardly.

22. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of eight inches provided with means to reduce a tendency for a longitudinal crack to form due to a substantial temperature difference between inner and outer portions of the electrode during or after use, said means including at least one generally radial slot extending longitudinally, said slot being deep enough for its side walls to be free to move toward and from each other due to temperature differences within the electrode, the depth of said slot being substantially less than the radial depth of the electrode exclusive of any core or bore, said slot extending for the full length of the electrode.

23. A cylindrical carbonaceous electric furnace electrode having a diameter in excess of eight inches providedwith means to reduce a tendency for a longitudinal crack to form due to a substantial temperature diiference between inner and outer portions of the electrode during or after use, said means including at least one generally radial slot extending longitudinally, said slot being deep enough for its side walls to be free to move toward and from each other due to temperature differences within the electrode, the depth of said slot being substantially less than the radial depth of the electrode exclusive of any core or bore, said electrode being provided with an axial bore and the slot opening radially inwardly into said bore.

CHARLES H. CHAPPELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 584,867 Foote June 22, 1897 1,058,057 Hinckley Apr. 8, 1913 1,065,141 Kerlin June 17, 1913 1,115,027 Seabury Oct. 27, 1914 1,313,126 Shoeld Aug. 12, 1919 2,527,294 Bailey Oct. 24, 1950 FOREIGN PATENTS Number Country Date 6,984 Great Britain Mar. 20, 1912 of 1911 6,986 Great Britain Mar. 14, 1912 of 1911 j i 456,341 Germany Feb. 18, 1928 

